1. Introduction
The invention relates to radiation sensitive compositions and, more particularly, to photoimageable compositions useful as soldermasks in the manufacture of printed circuit boards.
2. Background Art
Soldermasks, also known as solder resists, are compositions used to mask off a portion of a printed circuit board and prevent those areas from accepting solder during solder processing. See Coombs, Printed Circuits Handbook, ch. 16, McGraw Hill (3rd ed., 1988), incorporated herein by reference for its teaching of soldermasks and use thereof.
For years, screen printing was the means used to apply a soldermask to a substrate in the manufacture of printed circuit boards. It was a desired process because it permitted selection of masking materials for a variety of resins in the formulation of the soldermask and enabled one to apply a heavy coating on an uneven surface with a reasonable degree of accuracy. However, increased packing of conductor tracks and reduced track widths resulted in a demand for greater dimensional accuracy and resolution of features beyond the resolution capability of conventional screen printing masks.
A first step towards the formulation of a photoimageable soldermask was the introduction of screen printed photopolymerizable soldermasks utilizing acrylate resins. However, such photopolymerizable soldermasks generally did not gain significant market acceptance.
A next step in the evolution of soldermasks was the introduction of photoimageable soldermasks analogous to the photoimageable photoresists used in printed circuit board manufacture to define a desired conductor pattern. These photoimageable solder resists were generally solvent developable and employed acrylate-based resins. Two primary processes were used to apply these photoimageable soldermasks to a circuit substrate. One was as a dry film soldermask using a laminator to apply heat and pressure to the soldermask as it was applied to the circuit board substrate. The other method used a liquid soldermask, typically applied by means of screen or curtain coating equipment. Both processes possessed advantages and disadvantages. Disadvantages common to both included use of organic solvent development which can present disposal and other problems; poor to moderate adhesion to copper; and limited resistance to elevated temperatures. Poor adhesion to copper can result in delamination of the soldermask from the copper conductors during the soldering operation. Inadequate thermal resistance can result in deformation of the mask during soldering. Further, it was often difficult to generate fine lines of these solvent-developable solder resists.
More recently, aqueous developable soldermask compositions have been reported. These soldermasks are generally applied as a wet coating, such as by curtain coating, dried, exposed, and developed, typically with an aqueous alkaline solution.
Difficulties have been encountered in efforts to provide suitable aqueous developable photoimageable soldermask compositions. For example, shortcomings of prior aqueous developable photoimageable soldermasks have included failure to yield high resolution relief images upon development, reduced adhesion of composition coating layers to copper at elevated temperatures, poor thermal properties and/or poor resistance to processing chemicals such as fluxes.
It thus would be desirable to have a new photoimageable resist composition. It would be further desirable to have a new photoimageable composition that was aqueous developable. It would be still further desirable to have a photoimageable, aqueous developable composition that could be used as a soldermask.
In European Patent Application 0 425 142 A2, publication date Feb. 5, 1991, a positive-acting photoresist composition is reported which is said to contain at least one film-forming acid-hardening resin system, an acid or acid-generating material, preferably in the form of a thermal acid generator, and a photobase-generating compound. The radiation sensitive compositions of the present invention differ from this reported positive photoresist. For example, the compositions of the present invention suitably can be free or substantially free of an acid and/or acid-generating material.